1. Field of the Invention
This invention relates to a method of removing noble metal contaminants from a mineral acid bath, and more particularly, in the manufacturing of packaged semiconductor devices, a method for removing contaminant Po-210 from a heated phosphoric acid bath using silicon as a getter.
2. Description of the Prior Art
In the process of manufacturing large scale integrated circuits (LSI) from a silicon substrate or wafer, phosphoric acid is often used as a solution to selectively remove silicon nitride.
Phosphoric acid commonly contains trace element of Polonium-210 (Po-210). Po-210 is a radioactive element and a source of alpha radiation emissions. Levels of Po-210 and, in proportion, alpha emissions, vary depending on the content of Po-210 in phosphorous used to make the acid solution. The Po-210 present in any deposit of phosphorous is a function of the natural decay of U-238. Po-210 Levels may vary greatly from an imperceptible emission level to significant alpha emissions.
It is well know in the industry that alpha particle emissions are one of several known causes of soft errors in LSI memory devices. Soft errors have been defined to be random, non-recurring single bit errors in memory devices. They are not permanent, i.e., no physical defects are associated with the failed bit. A bit showing soft error is completely recovered by the following write cycle, for example, in a dynamic memory device where refresh of memory stored data occurs every several nanoseconds.
Other identified causes of soft errors are system noise, voltage marginality, sense amplifiers and pattern sensitivity, all statistical predictors of the rate at which soft errors will occur (SER).
Po-210, when present in phosphoric acid in the etching process, has been identified as a source of alpha particle emissions. Po-210 has an affinity for Silicon (Si) in the acid bath and plates onto the surface of the Si wafers which are being etched for production of finished LSI memory and other devices. The Po-210 remains bonded at surface sites on the silicon wafer, through later subsequent manufacturing steps. The finished LSI circuit or die results in a memory device with internally emitted alpha particles. The alpha particles are emitted by the Po-210 contaminating the die. Thus, the memory device becomes itself a source of contribution of SER.
Silicon is employed as a xe2x80x9cgetterxe2x80x9d to attract Polonium 210 (Po-210) molecules and remove them from the liquid phosphoric acid bath. Getter is used in this instance to describe silicon as the attracting agent which reacts with the noble metal, Po-210 in this example. The mechanism for this attraction is described as follows: The getter is the vehicle for removing the contaminating noble metal. Gettering refers to the act of attracting and removing the trace elements through the reduction of the trace elements onto silicon. Noble metals include silver, gold, copper, platinum, palladium, mercury, selenium and bismuth, in addition to Polonium.
In accordance with the teachings of the present invention, a method of removing noble metal trace elements from a mineral acid is disclosed, comprising the steps of heating a liquid mineral acid in a container, inserting a silicon getter into the acid in fluid contact with the acid, and removing the silicon getter from said mineral acid.
A method is set forth, wherein the liquid acid is heated to a temperature of approximately 145-150 degrees C. The mineral acid is selected from the group consisting of phosphoric acid or sulfuric acid, and any acidic solution of pH 6 or lower. The mineral acid contains at least a trace of a noble metal. The noble metal is selected from the group consisting of Polonium, gold, silver, platinum, copper, palladium, mercury, selenium and bismuth. The getter is inserted in the liquid mineral acid for at least thirty minutes.
A method of adsorbing Po-210 with a silicon getter from a phosphoric acid solution comprises the steps of providing liquid phosphoric acid in a container, heating the container of liquid phosphoric acid to between 145 degrees C. and 150 degrees C., inserting at least one silicon article into an open vessel, placing the vessel into the acid bath at least until reaching process temperature, soaking the silicon article in the liquid phosphoric acid and removing the vessel from the container.
An improved method of manufacturing electronic semiconductor integrated circuits is also disclosed. The method of substantially eliminating xcex1-particle emissions attributable to manufacturing materials, comprises the steps of providing a quantity of phosphoric acid having at least a portion composed of Po-210, then heating the quantity of phosphoric acid to a temperature suitable for removal of silicon nitride from a silicon wafer, inserting a silicon getter into the phosphoric acid during or following the heating step, next removing the silicon getter from the phosphoric acid, and etching silicon nitride from a semiconductor wafer having wiring paths defined thereon.
It is therefore an object of the present invention to provide a method of eliminating alpha particle emissions from phosphoric acid while simultaneously removing Po-210 from the acid solution.
It is further an object of the present invention to provide a method of decontaminating phosphoric acid for use in the manufacture of silicon LSI memory devices.
Another object of the present invention is to provide a method of plating noble metals onto silicon from an acidic solution.
Yet another object of the present invention is to provide a method of manufacturing a LSI memory device substantially free of soft errors due to internal alpha particle radiation.